For decades, Gunston Cove, a small tidal embayment off the Potomac River, was simply a mess. Its summertime water was painted green by algae blooms that long ago had smothered the last blade of underwater grass.

Amid campaigns to clean up the Potomac during the 1970s, phosphorus discharges from nearby treatment plants, which fueled the blooms, were slashed. By 1980, the amount of the nutrient entering Gunston Cove had been cut by 90 percent.

Not much happened in the cove, which is located on the Virginia side of the river near Mason Neck. In fact, by some measures, the mess got worse, as dense algae blooms continued to coat the summertime water.

Scientists scratched their heads. "There was kind of a shock that went through the Potomac community at that point," said Christian Jones, director of George Mason University's Potomac Environmental Research and Education Center. "You couldn't even account for all the phosphorus in the algae from nutrient loadings. There was no way."

Part of the reason for the lack of improvement surprised scientists: The large algae blooms on the surface increased the water's pH, making it more alkaline. As the pH rose, it drew stored phosphorus out of the sediment-providing fuel for more algae growth, and therefore even higher pH, which meant more phosphorus, and so on.

But there is some good news to this story. After nearly two decades of waiting, Gunston Cove is finally getting cleaner. Water is clearer, and underwater grasses have recently begun surging back.

Among the contributing factors, Jones said, is that some of the phosphorus long-stored in bottom sediments has finally been used up. Also, new sediment-with lower phosphorus concentrations-is slowly burying the old sediment.

The long, slow lessons learned in Gunston Cove may contain a message for the rest of the Bay. "The path of recovery may not be the same as the path of degradation," Jones said. "These systems can take a long time to respond, and there is a sequence of reactions that the ecosystem will go through."

That's a lesson underscored in a new report by a team of scientists which cautions that predicting how-and when-the Chesapeake Bay will recover is "not a trivial matter" and warns that the public and politicians may have unrealistic expectations.

The report, "Thresholds in the Recovery of Eutrophic Coastal Ecosystems," warns that as nutrient levels are reduced, the Bay and its tidal tributaries may respond in unexpected ways, with some areas such as Gunston Cove showing "stubbornness" to recovery efforts, while others rebound in a rapid burst. Some areas may be so degraded, they may need to be jump-started with other restoration actions, such as habitat restoration initiatives, to restore a healthy system. And some sites may not return to their former condition for the foreseeable future.

The reason, according to the report, is that when coastal ecosystems become severely degraded, as is the case with the Chesapeake Bay, they cross an ecological "threshold" after which recovery is not necessarily a straightforward proposition: A steady decline in nutrients entering the Chesapeake may not be matched by a steadily improving ecosystem with clearer water, more grass beds and smaller oxygen-starved "dead zones."

As was the case in Gunston Cove, recovery may be stalled by unforeseen complications. And as in Gunston Cove, recovery may come in a sudden flourish after years of waiting-not in increments.

"We do know enough to be certain that good things will happen if we reduce nutrient loadings," said Michael Kemp, a scientist at the University of Maryland Center for Environmental Science and a co-author of the report. "Once the Bay's restoration process begins, we don't really know the trajectory and the time course over which that recovery will occur."

The difficulty in predicting recovery is due to the complexity of the Bay. The "healthy" Bay ecosystem of the past not only had fewer nutrients, but was maintained by a host of ecological interactions that involved large underwater grass beds as well as well as large populations of oysters and other filter feeders. Those interactions provided what scientists call "positive feedbacks," which helped to maintain the health of the ecosystem.

Today, those positive feedbacks have been replaced by other reinforcing feedbacks that help to maintain the degraded system-such as what happened with the algae-pH-phosphorus cycle in Gunston Cove.

When a system goes past a threshold, it essentially becomes mired in an ecological rut, with the new feedbacks making recovery more difficult.

On a small scale, this is sometimes seen with underwater grass beds, especially in areas with lots of wave energy. If excess nutrients spur algae blooms that smother the grass beds, simply reducing nutrients may not cause the grasses to return. Why? The grass beds were modifying the local ecosystem by stabilizing the sediment-a positive feedback.

Even with reduced nutrient pollution, waves may still stir up enough sediment to cloud the water and prevent the return of grass beds. The loss of grasses, in effect, pushed the system over an ecological threshold by dramatically changing conditions in a way that maintains poor water quality. In addition to nutrient reductions, the path to restoration could now require extra efforts to control sediment and plant grasses.

In the Bay, scientists suggest, this type of alteration has happened on a grand scale. Instead of having feedback mechanisms that push water quality in the right direction, the Bay is filled with feedbacks that support a degraded system. For instance, the Bay's bottom sediment was once filled with a mix of worms, clams and other benthic dwellers that naturally removed nitrogen from the water and helped to store excess phosphorus in the sediment.

Today, the large oxygen-starved areas that cover much of the Chesapeake's bottom have wiped out those creatures in huge areas. Taking their place are bacteria, whose high metabolism rates draw oxygen out of the water, which enhances the availability of nitrogen and phosphorus to support more algal growth. In effect, many species that helped to improve water quality have been eliminated from key areas of the Bay.

But scientists say it was not a single factor that pushed the Chesapeake Bay over the edge.

Nutrients were increasing since the early 1900s, and underwater grass beds were declining since at least mid-century. Tropical Storm Agnes in 1972 buried huge areas with sediment, smothering much of the grass beds that remained and leaving the Bay awash in nutrients for years. In the late 1970s and 1980s, the oyster population plummeted because of disease, overfishing and habitat degradation.

In other words, said Denise Breitburg, a scientist with the Smithsonian Environmental Research Center, it was a "perfect storm" of stresses that worked together to send the Bay over a threshold.

The fact that so many factors have combined to alter the Chesapeake make recovery especially difficult. If the Bay were not overloaded by nutrients, it may have been able to bounce back from Agnes, Breitburg noted. Meanwhile, the loss of oysters means the loss of their filtering capacity that once helped clear the water of sediment and algae.

"We've changed the baseline that we're working from," Breitburg said. "Reducing nutrients back to the point when we had really abundant oysters in the Bay may not get us to the same place as we were when oysters were there."

Based on local conditions, some places may recover differently, and faster, than others.

The varied routes that recovery may take can be seen in a single Bay tributary, the Patuxent River.

In the mid to late 1980s, wastewater treatment plants on the river started removing phosphorus from their discharges. "Frankly, we didn't see much of a response," said Walter Boynton, a scientist with the University of Maryland Center for Environmental Science who has long studied the river.

But in the early 1990s, when treatment plants started to also remove nitrogen, water quality responded "in the blink of an eye," Boynton said. Algae concentrations in low- and mid-salinity portions of the tidal river quickly declined, and underwater grasses soon began sprouting.

The recovery was both a bit of a surprise, and reassuring. The surprise was that the low-salinity areas responded so much to reduced nitrogen loads-typically phosphorus is more important in low-salinity water, while nitrogen is important in higher salinities. (In Gunston Cove, also a low-salinity area, changes in nitrogen concentrations seemed to have no impact on algae production.)

Yet the Patuxent also proved that large actions-nutrient levels from discharges were cut roughly in half-can in some cases provide quick results. "That is the simplest form of response that we could hope for," Boynton said. "That is, you do something that is of significant size, and you get a response quickly and in the desired direction.

"That's not going to be true across the board," he added. "I wish it was, because it is so simple."

In fact, even in the Patuxent the story gets more complex farther downstream, as salinities rise. In high-salinity portions of the river, there have been no reduced algae concentrations and no recovery of underwater grass beds. In deeper water, chronic low-oxygen conditions persist through the summer. In that area, scientists say nutrient reductions upstream might be offset by nutrient-rich water being pushed by tides from the mainstem Bay into the river.

Other factors may come into play as well, such as the loss of oysters.

The demise of oyster reef habitat limits the solid surfaces available for for sea nettle polyps, the bottom-dwelling stage of the sea nettle's life cycle. Without enough substrate, the sea nettle population declined. The decline of sea nettles led to an increase in comb jellies, which eat oyster larvae, keeping them from rebuilding.

One reason the high-salinity areas of the Patuxent River may not be recovering, the report suggests, is that solid substrates fell below a critical threshold in the mid 1980s, essentially keeping oysters from playing a significant ecosystem role.

"The sea nettle-oyster link in the Bay's food web is reinforced by trophic interactions that are effectively stuck in a rut," the report said. "Fewer oysters mean fewer nettles, fewer nettles mean many comb jellies, many comb jellies mean fewer oyster larvae, fewer oyster larvae mean fewer oysters."

Pushing systems past a new tipping point that restores positive feedbacks that enhance water quality will likely require big, not incremental, efforts, the report said. On top of the "to do" list is nutrient reductions.

"There is a great deal of field and laboratory evidence to suggest that the first step in this process of restoration is getting nutrient loads down, cutting them by about half," Boynton said. "That's a very, very old story, but it seems to me to be roughly correct."

That, he said, would restrict the oxygen-starved portions of the Bay and tidal rivers to a much smaller area, allowing the return of healthy benthic communities that would provide the positive feedbacks that help-rather than hurt-recovery efforts. "Those are the kinds of things that can happen fast if you can restrict the amount of hypoxic water, and that seems to be related to nutrient loads," Boynton said.

But areas that have lost seagrass beds or other important habitats that played important roles in maintaining water quality may not regain biological feedbacks without a jump-start in the form of strategic restoration efforts to get them out of the rut.

"When we are talking about thresholds for recovery, we are talking about accumulating a critical mass of positive changes that will hopefully cause recovery to happen in a sudden burst, and push feedbacks in a more beneficial direction," said Erica Goldman, a scientist with Maryland Sea Grant and co-author of the report.

Kemp said scientists and agencies need to pick the areas most likely to respond quickly-mostly shallow, low-salinity areas of tidal rivers and the Bay-and target restoration activities.

And, he said, they need to experiment with new approaches to restoration, as most efforts to bring back grass beds and oyster reefs have failed over the long term. For instance, he suggested that underwater grass restoration efforts might be targeted for years when long-term forecasts predict drier-than-normal weather to take advantage of optimal growing conditions. The goal would be for the grasses to have enough time to become firmly established before wet conditions return-and to be large enough to withstand those conditions.

And while bringing back oysters may take decades, he said efforts could be made to plant clams in newly planted grass beds to help jump-start biological water-filtering capacity. As lessons are learned from such projects, he said, they could be applied in other areas.

The challenges posed by the need to overcome thresholds creates a public relations problem, the report notes. While the public is asked to support a cleanup that costs billions of dollars, it might not have not been adequately warned that the recovery may not be obvious until sometime-potentially many years-after nutrient reduction goals are achieved.

To the contrary, the report said, people have been given a message that "we know how to restore the Bay, but we simply lack the resources to do it. This view may have oversimplified our understanding of precisely how and when the ecosystem will respond to decreases in nutrient loads."

It's a concern shared by some in management agencies. Rich Batiuk, associate director for science with the EPA's Bay Program Office, said it was important that the public understand that the Chesapeake may be slow to respond to cleanup efforts.

Besides better targeting restoration actions, he said, work is needed to identify subtle changes that may serve as early indicators that the Bay is responding. "We need to set the expectations better about what the public could anticipate on the road to recovery," Batiuk said. "It's not a movie going in reverse."

After all, the most important threshold in Bay restoration could be flagging public support. "According to what we know, and what we've seen in other situations, you can go for long periods with nutrient reductions and nothing happens," Kemp said. "Then all of a sudden, it starts happening very, very rapidly. We are trying to preach a little bit of patience."

To download a copy of Thresholds in the Recovery of Eutrophic Coastal Ecosystems," a joint publication by Chesapeake Bay Program's Scientific, Technial and Advisory Committee, and Maryland Sea Grant, visit www.mdsg.umd.edu/issues/restoration/resilience/thresholds/.

Dearth Of Estuary Case Studies Make Predicting A Timetable Even Murkier

Predicting how the Bay and its tidal tributaries will respond to restoration efforts is problematic because of the lack of examples from other areas.

Most studies of nutrient impacts on water bodies-or eutrophication-have taken place in freshwater lakes and ponds. When enriched with phosphorus, algae concentrations increase and water quality worsens; the food web changes; underwater plants disappear; and oxygen concentrations decline. When phosphorus levels are cut, the lakes recover. The path to recovery in shallow and deep lakes is somewhat different, but after several years of reductions, both show marked improvement.

In estuaries, the story is much more murky. They have changing salinities, two nutrients (nitrogen and phosphorus) to deal with, more complex water circulation patterns, and often great changes in depth. Further, they often have complex food web interactions which greatly affect how the system responds to nutrient enrichment and cleanup efforts.

Unraveling those interconnections is difficult, in part, because coastal systems, in general, have not been studied as long as or as intensely as lakes.

"There are a lot of systems where we have done a really good job of reducing phosphorus loadings," said Denise Breitburg, a scientist with the Smithsonian Environmental Research Center. "But the number of saltwater systems that have dramatically reduced nitrogen loadings, and reduced them to the point where you really expect to see a response, are far fewer."

Those that exist offer a mixed bag of results.

In Tampa Bay, sharp nitrogen and phosphorus reductions resulted in reduced algae concentrations and a rebound of underwater grasses, although the grass rebound lagged about a decade behind the nutrient reductions.

But in Denmark, which has had a national strategy to control nutrients entering all waterbodies since 1987, freshwater rivers and lakes have seen lower nutrient concentrations, less algae, clearer water and improved fish community structure. Danish coastal waters have also had reduced nitrogen and phosphorus concentrations, but signs of significant ecosystem improvement-except for better water clarity-are yet to be seen.

Michael Kemp, a scientist with the University of Maryland Center for Environmental Science, said those and other case studies tend to suggest that shallow, low-salinity areas-such as the upper parts of the Bay and tidal rivers-are often those most responsive to restoration efforts.

Around the Bay, those areas are typically influenced by phosphorus, which has been more successfully controlled than nitrogen. Populations of filter feeding clams, and zooplankton-microscopic consumers of algae-also tend to be more abundant in low-salinity water.

"There are a lot of factors, not just one or two, that make the upper regions more amenable to getting positive feedback processes fired up and working for you," Kemp said.